Means for selecting electron beam energy



Nov. 28, 1961 R. F. POST MEANS EOE SELECTING ELECTRON BEAM ENERGY Filed NOV. 29, 1957 INVENTOR. FMH/4,@ F P057' :EIS- 2 BY I @MD- Hman #Tram/Ey United States Patent O 3,011,086 MEANS FOR SELECTING ELECTRON BEAM ENERGY Richard F. Post, Walnut Creek, Calif., assigner to Applied Radiation Corporation, Walnut Creek, Calif., a

corporation of California Filed Nov. 29, 1957, Ser. No. 699,744 4 Claims. (Cl. S15- 5.35)

The present invention relates to a method and means for the optimum selection of electron beam energy, and more particularly to the production of a variable current high energy electron beam in a linear accelerator.

The present invention contemplates the production of a velocity modulated bunched beam of electrons which includes alternate high and low electron density portions, the low density portion being commonly termed back phase or voltage doubled electrons and having substantially greater energies than the high density or inphase portion of the beam. Low energy electrons are then rejected and high energy electrons are preferentially selected to thereby' result in the production of successive bunches of high energy electrons devoided of substantially all of the in-phase electrons of the original beam and wherein only a selected high energy portion of the original voltage doubled electrons are present. The selected electron energy is essentially controllable over a substantial range and therefore the energy ofthe bunched electrons is variable as well as the amount or current thereof. The energy of the rejected in-phase electrons may then be extracted and employed in proper phase relationship to accelerate the selected high energy bunches of electrons to even higher energies as for example in various electron linear accelerators. In this connection, it is important to note that the method and apparatus of the present invention may be advantageously employed to vary the injector current and therefore of the power output of the accelerator continuously over a wide range, which function has not been heretofore achievable in conventional electron linear accelerators.

It is therefore an object of the present invention tto provide a method and means for controlling the beam current and power output of an electron linear accelerator.

lt is another object of the invention to provide an improved method and means for producing high energy electron beams.

Still another object of the present invention is the provision of a method and apparatus for achieving optimum rejection of low energy electrons and preferred selection of high energy electrons in a velocity modulated bunched electron beam.

It is a further object of the present invention to provide a method and means for rejecting low velocity electrons of a velocity modulated electron beam while adjustably selecting the high velocity electrons therefrom for injection into a linear accelerator waveguide, the energy of the rejected low energy electrons being converted to ratio frequency energy and coupled in proper phase relationship to the waveguide to accelerate the injected high velocity electrons.

The method and means of the present invention is herein illustrated and described with respect to specific steps and structure in the interest of clarity, however, no limitations are intended or to be inferred therefrom, reference being made to the appended claims for a precise delineation of the scope of the invention.

The invention is illustrated in the accompanying drawing wherein:

FIGURE 1 is a schematic representation of a preferred embodiment of the beam energy selector of the present invention as employed in one type of electron linear accelerator, and

31,011,086 Patented Nov. 2%, i961 lCe FIGURE 2 is a graphical illustration of typical magnetic fields as related to the embodiment of FIGURE l.

Considering now the invention in some detail as to the method thereof, it is contemplated that there will be produced a bunched velocity modulated beam of electrons having periodic electron density variations separated in phase by The bunched beam is then employed to produce electromagnetic energy as bypassing the beam through a resonant cavity whereby same is excited. In addition certain relatively few electrons in the bunched beam attain substantially twice the velocity lor voltage of the remaining electrons in passing through the cavity 180 out of phase with reference to the phase of driven oscillations in the cavity. There thus results a bunched electron beam having alternate high and low density bunches of, respectively, relatively low energy in-phase electrons and high energy out-of-phase electrons, and the simultaneous production of electromagnetic radiation. An optimum high energy portion of each high energy electron bunch is next controllably selected and the remaining lower energy electrons are rejected while the electromagnetic radiation is employed to further accelerate the optimum high energy electrons and return energy thereto. There is accordingly produced by the process of the present invention, a resultant extremely high energy electron beam, the energy, power, and current of which is adjustable by virtue of the controllable energy selection of the electrons accelerated.

'It will be appreciated that the bunched electron beam and simultaneous production of electromagnetic radiation produced in the first portions of the above process may be produced in the output cavity lof a conventional klystron or equivalent means. In a klystron, however, the bunched electron beam is conventionally collected by a collector beyond the output cavity resulting in the production of only electromagnetic energy. The output cavity of the klystron may be apertured or otherwise modified, however, to permit passage of the bunched electron beam therefrom. The beam may in addition be collimated or otherwise operated upon to materially reduce the beam current in passage out of the output cavity whereby the main portion of the beam is defocused in the process of establishing the electromagnetic energy. The collimation preferably discriminates in favor of the voltage doubled ror out-of-phase electrons.

Of primary importance in the collimation process of the present invention, beam electrons having energies below an adjustable predetermined level are optimurnly rejected whereas beam electrons having energies above said level are selectively transmitted from the cavity and may be subsequently accelerated as described khereinafter. More particularly, there is established an axially symmetric magnetic eld which converges radially inward toward the axis of propagation of the bunched electron beam in the region of the output cavity. The field lines then become parallel and diverge radially outward from the beam axis. Such iield is effective in collimating the electron beam lin the converging and parallel eld regions. The diverging field region however is effective in blowing-up the beam, i.e., causing the beam electrons to deviate uniformly outwardly from the axis. Moreover, the amount of deviation of each electron is inversely related to its energy. The lower energy electrons accordingly deviate from the longitudinal direction by a substantially greater amount than the higher energy electrons which tend to center closely about the axis. The selector tield thus discriminates between the electrons with respect to their energies.

The foregoing may be explained by noting that where magnetic lield lines curve, the electrons can be considered as possessing components of velocity that are respectively parallel to, and at right angles to, the direction of the trons to spiral tangent to an axis delined by the magnetic field line. The velocity component in the direction of the field, however, is not affected by the presence of the field. Thus as the magnetic iield curves away from the initial direction of travel of the electron beam, i.e., along the beam axis, each electron follows a helical path of such a character that once each revolutionof the helix, the electron becomes tangent to the same linx line. The radii of the helical paths of the electrons is small for low energy and large for high energy electrons. The low energy electrons thus follow the magnetic field lines closely whereas the high energy electrons deviate therefrom due to their large helical radii. Accordingly, in the diverging region of the magnetic field where the field lines curve radially outward, the low energy electrons closely following such field lines, diverge radially outward also. The high energy electrons in deviating from the curved field lines are accordingly centered closely about the axis of beam translation. The energy distribution of the beam electrons with respect to beam radius in the diverging field region is thus generally characterized by a decrease inelectron energy with increasing radius.

The previously mentioned predetermined electron energy level which determines the minimum energy of the range of the high energy electrons selected for transmission in the process of the present invention may accordingly be determined by trapping all electrons Vdeviating from the direction of beam translation by amounts greater than the amount of deviation corresponding to the lpredetermined energy level. Such trapping is preferably accomplished by intercepting the critically deviating beam electrons as for example upon a coaxially arranged annular collecting surface which permits uninhibited' passage axially therefrom of the relatively high energy electrons having deviations less than the critical amount. Moreoventhe Iintensity of the selector field may be varied to in turn vary the amount of electron deviation whereby the energy range of electrons intercepted by a,

particular ixed collecting surface is similarly variable. The axial position of the selector field may in addition be varied with respect to the collecting surface to adjust the energy discrimination level.

Considering now the acceleration of the selected high energy portion of the hunched electron beam by the electromagnetic energy generated in the early steps of the process, it is to be noted that several alternative steps are possible.V The electromagnetic waves may be coupled to the selected electron beam :with the phase of the former varied with respect to the beam pulses, or vice versa. In order to obtain the proper phase relation, the phase of the electromagnetic waves may be for example retarded with reference to the beam pulses. The beam pulses may then be passed through the electromagnetic waves in phase therewith, Power losses as would result from acceleration of out-of-phase electrons are thus eliminated. Retarding of the energy wave phase may be accomplished as by directing the energy waves through a variable length transmission line wherein a predetermined length of line shifts the phase of the electromagnetic waves by the desired amount. Such waves emerging from the line are coupled to the beam output from the collecting surface in proper phase relationship with the selected high energy electron beam to accelerate same.

There is thus produced by the above method a high voltage electron beam of adjustable energy and beam intensity. Various structural arrangements may be employed to carry out the described method and referring to FIGURE l of the drawing there is illustrated one preferred means which is embodied as a unique linear electron accelerator wherein the output power, beam current intensity, and energy are variable over a wide range. As shown in the figure, there is provided a velocity modulated bunched electron beam source y1v1 axially aligned with a waveguide 12 and communicating therewith through a beam energy selector and RF coupler 13. Electron source y11 may be, for example, a conventional electron-beam radio-frequency tube such as a klystron. Such tube generally comprises a thermionic emitter 14 for emitting electrons within the tube, which electrons are commonly accelerated through an apertured anode 16 which is externally maintained at a relatively more positive potential than the emitter. A continuous beam of electrons thus .emerges from the apertured anode 16. The continuous electron beam is passed through a buncher cavity 17 aligned with the emitter `and adapted for excitat-ion by an externally applied radio frequency voltage. The electron beam is thereby velocity modulated by the radio frequency voltage in a-conventional manner. Following buncher cavity 17 there is provided an output cavity 18 which -is designed to resonate at the frequency of the modulated electron beam and is coupled to the buncher cavity as by means of a drift tube 19. VSuch drift tube 19 is conventionallyencircled by a direct current energized solenoid 21 lfor collimating the electron beam during passage through the drift tube. The electron beam in traversing the relatively electric eld free space of drift tube 19 becomes hunched with periodic variations in electron density. More specifically, the bunched electron beam is of the character hereinbefore described, i.e., a series of alternate high density relatively low energy in-phase electron bunches and low density relatively highV energy out-of-phase electron bunches.

The various components and operation of the hunched beam source 11 described above are conventional in tubes such as klystrons. The hunched elec-tron beam in its entirety would be intercepted in a conventional klystron by a collector beyond output cavity 18, and a resonant exchange of energy would occur within such cavity resulting in the production of electromagnetic energy therein. The foregoing description is accordingly included herein merely -to illustrate but one possible means for producing a hunched electron beam of the character employed in the present invention.

Continuing now with a'description of the more salientI features of the invention and considering the hunchedV electron beam source 11 to be a conventional velocity modulated tube, it is to be noted that such tube is necessarily modified in the output cavity 18 thereof to permit passage of the hunched electron beam therefrom in addition to the production of electromagnetic energy. To facilitate the foregoing, the collector conventionally disposed beyond the output cavity of a velocity modulated tube is dispensed with and a central axial aperture 22 is provided in the output cavity. Aperture 22 thus facilitates coupling of the electron beam from cavity 18 to the beam energy selector and rad-io frequency coupler 13.

More particularly, coupler 13 includes energy discrimination means 23 for selecting relatively high energy portions of the beam emerging from the cavity exit aperture 22 for further transmission while intercepting and collecting the remaining low energy portions thereof. Such discrimination means 23 preferably includes a coaxial collector 24 in axial communication with exit aperture 22. Collector 24 comprises a hollow elongated cylindrical inner member 26 which is coaxially communicating with aperture 22 and inwardly tapered in a direction away from the aperture. A similarly tapered outer cylindrical member 27 is coaxially disposed about inner member 26 and is secured to cavity 18 in coaxial relationship with aperture 22.

The decreased. diameter ends of collector inner and outer members 26, 27 respectively are terminally coupled to inner and outer members 28, 29 of a cylindrical coaxial feed tube 31 terminating at the input end of accelerating waveguide 12. The reduced exit opening 32 of member 25 thereby comprises the beam injector hole to waveguide 12.

Energy discrimination means 23 further includes an energy selection magnet 33 concentrically disposed with reference to the axis of collector 24 and axially translatable with respect thereto. Magnet 33 may be for example, a hollow cylindrical permanent magnet, or more preferably a solenoid 34 coaxially encircling collector outer member 27 and energized by a variable D.C. power supply 36. Magnet 33, whether a permanent magnet or solenoid 34, may be rendered axially translatable by conventional means such as lead screws or the like (not shown).

Magnet 33 accordingly produces a magnetic eld 37 of the character hereinbefore described with respect to the process` of the invention within collector inner member 26 as illustrated in FIGURE 2. As shown therein, eld 37 is axially symmetric and converges radially inward in a region 38 proximate output cavity 1S and thence diverges radially outward in a region 39 proximate beam injector hole 32. Within output cavity 18 there is also established a diverging eld region 41 due to the klystron drift tube solenoid 21. Thus the bunched electron beam expands radially in the region between output cavity 18 and collector 24 due to the fringing portions of the klystron eld 41. A substantial portion of the relatively low energy electrons of the bunched beam, particularly many of the in-phase electrons', thereby impinge upon the walls of the collector resulting in the production of electromagnetic energy or waves. The relatively high energy electrons including a portion of the in-phase electrons` and substantially all of the out-of-phase or voltage doubled electrons of the bunched beam, however, encounter the converging portion of energy selector eld 37 in region 38 before they can impinge upon the walls of cavity 18. Such high energy electrons are prevented from reaching the cavity walls due to the energy selector field and are funneled or collimated by same through aperture 22 and inner member 26 of collector 24. The electron beam in transit through inner member 26 is restricted to the central axial regions thereof due to the collimating action of the field. Upon encountering the diverging portion of field 37 in region 39, the beam expands in the manner previously described in detail with respect to the process of the invention, i.e., the electrons deviate outwardly from the longitudinal direction by amounts which are dependent upon their energies. The electrons having energies less than a level determined by the intensity of selector field 37 and axial position thereof with respect to collector 24 impinge upon the walls of collector inner member 26 and tne walls of ytransfer tube 28 since they closely follow the diverging field lines. The electrons having energies greater than such level tend to deviate from the diverging field lines and follow substantially longitudinal paths extending through beam injector hole 32 into inner member 28 of coaxial feed tube 31. Therefore the energy of the beam entering the feed tube through the injector hole may be varied by appropriate adjustment of power supply 36 and/or the means for translating solenoid 34 axially of collector means 24.

n order that Vthe selected energy portion of the electron beam is accelerated by the electromagnetic energy generated in output cavity 18, means are provided for coupling such electromagnetic energy to coaxial feed tube 31 in proper phase relationship with respect to the beam to accelerate same. The foregoing is best facilitated by means of a variable length transmission line 42 connected between output cavity 18 and feed tube 31. The length of line 42 may consequently be adjusted to delay the phase of the electromagnetic waves emanating from cavity 18 and coupled to the feed tube by the exact amount necessary to effect acceleration of the beam in waveguide 12 in the well known manner.

More particularly, transmission line 42 is preferably constructed as a U-shaped coaxial coupler coupled at one end to cavity 18 as by means of a conventional inductive pick up loop 43 and at the other end to feed line 31 as by means of a coaxial T connection as shown generally at 44. The length of line 42 may be varied over a. wide range by inserting graded length insert sections therein. It is sometimes more advantageous however to insert conventional flexible bellows in the line to facilitate length variation.

IIt will be appreciated that the impedance of transmission line 42 varies with changes in its length. Therefore, a suitable variable impedance-matching system is provided to match the impedance of cavity 118 and fee-d tube 31 to the impedance of transmission line 42, no matter what its length. One suitable system which may be employed is a conventional double-stub impedance-matching system as shown in the drawing. Such system comprises a pair of shunting stubs 46, 47 in axial alignment with the two parallel side sections of transmission line 42. Annular short circuiting plungers 48, 49 slideably engaging the inner and outer conductors of stubs 46, 47 respectively, render such stubs adjustable in length by variation of the axial positions of the plungers therein. The lengths of the stubs 46, 47 may thus be varied to match the impedance of transmission line 42 at any length.

Operation of the above described apparatus follows from the previously described method of the invention. A bunched electron beam having spaced high and low electron density portions is established by source 1'1 and enters the output cavity 18 thereof which is designed to resonate at the frequency of the beam density variations. The beam bunches in traversing output cavity 18 give up energy to the cavity to establish and sustain electromagnetic oscillations therein due to the relatively low energy portions of the beam impinging upon the cav-ity walls. The remainder of the beam then passes through cavity exit aperture 22 into collector inner member 26 of coupler 13 and is therein axially collimated by the energy selector field 37 established by solenoid 34. The axial position of solenoid 34 vand the current output of power supply 36 are then appropriately adjusted to establish a predetermined minimum electron energy injection level determined by the intensity and axial position of the field with respect to collector 24. All beam electrons having energies less than the desired minimum injection level are rejected by the field in the manner hereinbefore described. All beam electrons having energies greater than said level are transmitted through beam injector hole 32 into coaxial feed tube 31 of accelerating waveguide 12.

The electromagnetic energy generated in output cavity 118 is also coupled into feed tube 31 through variable length transmission line 42. The length of such transmission line is adjusted to delay the phase of the elect-romagnetic waves with respect to the beam injected into waveguide 12 by an amount whereby the electromagnetic Waves are effective in accelerating the injected electron beam through the waveguide. There is thus produced at the output of waveguide 12 a high energy bunched electron beam, the energy of which is adjustable over a substantial range of means of selector eld solenoid 34. Moreover, inasmuch as the quantity of injection electrons projected through beam injector hole 32 into accelerating waveguide 12 may be continuously controlled by means of soelnoid 34, the beam current and therefore the power output of the accelerating waveguide is similarly variable over a substantial range, which feature has not been heretofore achievable in conventional electron linear accelerators.

As regards the calibration of variable current supply 36 and axial position of solenoid 34 with respect to output beam energy, current -and power, it will be appreciated that such calibration may be derived through rigorous mathematical formulation. In practice, however, it is usually more advantageous to measure the above noted output beam characteristics as the solenoid current and position are varied over a wide range in order to obtain a series of values which may be ploted to provide reliable calibration characteristics. Similarly, with respect to the length of transmission line 42 which delays the phase of the electromagnetic energy by an amount producing optimum acceleration of the beam, such length is best determined through experimentation. For example, with the current and position of solenoid 34 maintained at constant values, the output beam energy may be observed at various lengths of transmission line 42 to determine the length of line which produces the desired beam energy.

There has been described above an improved method and means for electron acceleration wherein the beam energy, current, `and power are continuously adjust-able over wide ranges. While the invention has been described with respect to specific steps and structure, it will be apparent to those skilled in the art that numerous variations and modiiications may be made within the spirit and scope of the invention, for example, the radio frequency coupler 13 including collector 24 and the feed tube 31 may comprise rectangular or other shaped cross section wave guides as well as the cylindrical coaxial conjigu-rations therefor as described hereinbefore and as illustrated in the drawing. Thus, it is not intended to limit the invention except as dened in the following claims.

What is claimed is:

1. An improved electron accelerator comprising a velocity modulated hunched electron beam source, an open ended resonant cavity coupled in axial alignment with said source for producing radio frequency energyV and permitting traversal by the hunched beam generated by said source, an accelerating waveguide disposed in axial -alignment with the open end of said cavity, a beam energy selector coupling said cavity to said waveguide, said selector having magnetic means for discriminating the beam electrons with respect to energy and collector means for intercepting electrons having energies below an adjustable predetermined level while transmitting the remaining relatively high energy electrons therefrom axially into said waveguide, and radio frequency coupling means connected between said cavity and waveguide for energizing said waveguide with said radio frequency energy to accelerate the relatively high energy electrons, said beam energy selector comprising a hollow coaxial tubular collector coupled between said resonant cavity and said waveguide, said collector tapered radially inward in a direction toward said waveguide, and means establ-ishing an axially symmetric magnetic eld longitudinally of said collector, said field converging toward the axis to become parallel therewith and then diverging radially outward from the axis within said collector.

2. In an injector for an electron linear accelerator including a periodically loaded waveguide, the combination comprising a klystron having an open-ended output cavity, a coaxial tapered tubular collector connecting said klystron and waveguide in axial alignment with said output cavity communicating with said waveguide, a solenoid disposed coaxially about said collector, said solenoid axially moveable with respect to said collector, a variable,

direct current power supply connected in energizing relationship -to said solenoid whereby the axial position of said solenoid and output of said power supply are adjusted to vary the energy range of high energy electrons injected into said waveguide, and a variable length transmission line coupled between said klystron output cavity and said waveguide for shifting the phase of electromagnetic energy radiated from said klystron by a predetermined amount to eiect acceleration of said high energy electrons in said waveguide.

3. In an injector for an electron linear accelerator as defined 'by claim 2, the combination further defined by said variable length transmission line being varied in length by a plurality of graded length insert sections and including a double-stub impedance-matching system.

4. An improved electron linear accelerator comprising a klystron having an open-ended output cavity and including a drift tube magnet producing a fringing magnetic field in said cavity, a periodically loaded waveguide having a coaxial feed tube, a coaxial hollow cylindrical collector communicably connected between said klystron output cavity and said coaxial feed tube, said collector inwardly tapered in a direction toward said waveguide and forming a reduced diameter beam injector hole with said feed tube, a solenoid disposed coaxially about said collector and axially moveable with respect to said collector, a variable direct current power supply connected in energizing relationship to said solenoid, a U-shaped coaxial directional radio frequency coupler connected between said klystron output cavity and said waveguide coaxial feed tube, said coupler variable in length, a pair of coaxial shunting stubs connected in axial alignment with the parallel side sections of said U-shaped coupler, and a pair ofY annular plungers slideably engaging the inner and outer conductors of said pair of coaxial stubs respectively for matching the impedance of said coupler to that of said cavity and said feed tube whereby a bunched beam of electrons of selected energy is injected through said beam injector hole into said waveguide and accelerated therein by electromagnetic waves generated in said klystron output cavity and coupled to said Waveguide through said coaxial coupler.

References Cited in the file of this patent UNITED STATES PATENTS 2,157,585 Zworykn et al. May'9, 1939 2,222,902 Hahn Nov. 26, 1940 2,272,165 Varian et al. Feb. 3, 1942 2,295,315 Wolff Sept. 8, 1942 2,305,884 Litton Dec. 22, 1942 2,306,875 Fremlin Dec. 29, 1942 2,450,602 Levialdi Oct. 5, 1948 2,473,031 Larson June 14, 1949 2,480,133 Hansen Aug. 30, 1949 v2,498,886 Hahn Feb. 28, 1950 2,532,7916 Warnecke et al Dec. 5, 1950 2,813,996 Chodorow Nov. 19, 1957 2,922,921 Nygard Jan. 26, 1960 2,940,000 Geisler June 7, 1960 FOREIGN PATENTS France Mar. 18, 1957 

